JANUARY 2021 EVENT (WED 13 JAN, 2 PM EST/11 AM PST)

COMPUTATIONAL EXPLORATIONS OF SOME NOVEL GROWTH MECHANISMS LEADING TO PAH AND PANH SPECIES – Martin Head-Gordon (Kenneth S. Pitzer Distinguished Professor of Chemistry, University of California, Berkeley)

There have been significant improvements in the ability of density functional theory (DFT) methods to accurately model chemical reactions over the past 5 years. These improvements will be summarized, and the resulting methods applied to address some interesting pathways towards the formation and growth of aromatics under conditions relevant to the interstellar medium. The reactions considered will include viable routes to formation of benzene from acetylene clusters or ices under ionizing conditions, and the corresponding pathways to pyridine (with relevance to the origin of DNA bases) in the presence of both acetylene and hydrogen cyanide. If time permits, ionization-induced bond formation between aromatics and pyridine will also be considered to unravel the nature of this exotic strong interaction.

PRESTELLAR PROVENANCE OF COMET 67P/CHURYUMOV-GERASIMENKO – Maria Drozdovskaya (Center for Space and Habitability, University of Bern)

Our Solar System harbors several thousand known comets with many more awaiting discovery. Comets are thought to be some of the most pristine relics that survive to this day, which may shed light on the volatiles and refractories that nourished our infant Solar System. In my talk, I will discuss the results from the ESA Rosetta mission that were obtained during its two year-long escort of comet 67P/Churyumov-Gerasimenko. I will address the story told by the comet's volatile inventory and deuteration, as deduced by the ROSINA instrument. The connections with star-forming regions, such as the low-mass IRAS 16293-2422, will be explored. In my talk, I will present evidence that leads to the conclusion that comets are capable of revealing the full evolutionary scenario of the low-mass system that we call home.

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DECEMBER 2020 EVENT (WED 9 DEC, 2 PM EST/11 AM PST)

THE UNEXPECTED CHEMISTRY IN PLANETARY NEBULAE: FROM CO TO C₆₀ – Lucy M. Ziurys (Regents Professory, CBC and Astronomy, University of Arizona)

For decades, planetary nebulae (PNe) have been considered to have limited molecular content, confined mostly to diatomic molecules. The strong ultraviolet radiation field generated by the central white dwarf star was thought to destroy molecules readily generated in the prior Asymptotic Giant Branch (AGB) phase. Theoretical calculations verified this line of thought. Over the past several years, however, we have been conducting observations of various molecules in PNe at millimeter wavelengths. We have now detected polyatomic molecules such as HCN and HCO⁺ in over 30 PNe, and species as complicated as CH₃CN, H₂CO, and c-C₃H₂ in several select nebulae. Furthermore, the molecular abundances do not appear to significantly vary with the age of the nebulae over the 10,000 year PN lifetime. Circumstellar abundances appear to be principally altered in the protoplanetary nebulae (PPNe) stage. Such molecular material must seed diffuse clouds, accounting for the polyatomic species observed there. Notably, C₆₀ is also observed in some PNe. We have also conducted recent solid-state laboratory imaging and spectroscopy, as well as molecular dynamics (MD) simulations, that suggest that C₆₀ is formed in the PPNe phase from the destruction of silicon carbide (SiC) grains.

ASTROCHEMICAL FORECASTING WITH MACHINE LEARNING – Kelvin Lee, (Department of Chemistry, Massachusetts Institute of Technology)

Since the first molecules were detected in space, we have now reached a point where chemical and physical complexity in the interstellar medium reaches the boundaries of what human expertise and intuition alone can achieve. With every new molecule we discover, the question "What comes next?" grows more and more difficult to answer as more possibilities emerge. Conventionally, we turn to chemical models for guidance; this may be complicated when considering complex, non-LTE processes such as shocks, radiation, and grain-surface chemistry. Moreover, expansion of chemical networks typically requires hand-picked reactions and species, requiring an exhaustive knowledge of chemical and astrophysical literature, and can impose human bias on which reactions and molecules are important. As a complimentary approach to conventional chemical models, we have developed an unsupervised machine learning pipeline for predicting molecular abundances in a non-parametric fashion. Leveraging tools originally developed in high throughput drug discovery and data science, our pipeline captures and uses millions of molecules from various databases to create chemically descriptive vector representations for quantitative comparison. These representations are subsequently used to predict molecular properties in a given environment; as a proof-of-concept, we use the well-characterized chemical inventory of TMC-1, including the latest discoveries from the GOTHAM collaboration. We show that the model can be successfully conditioned on an inventory, able to reproduce column densities of unseen molecules to within an order of magnitude without any tuning parameters. Simultaneously, we are able to use the model to predict column densities of hundreds of thousands of molecules not yet detected in space, as a way to guide efforts, as well as provide a robust statistical baseline for expected abundances.

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NOVEMBER 2020 EVENT (WED 11 NOV, 2 PM EST/11 AM PST)

PREBIOTIC ASTROCHEMISTRY IN THE "THz-GAP" – Susanna Widicus Weaver (Department of Chemistry, University of Wisconsin - Madison)

Small reactive organic molecules are key intermediates in interstellar chemistry, leading to the formation of biologically-relevant species as stars and planets form. These molecules are identified in space via their pure rotational spectral fingerprints in the far-IR or terahertz (THz) regime. Despite their fundamental roles in the formation of life, many of these molecules have not been spectroscopically characterized in the laboratory, and therefore cannot be studied via observational astronomy. The reason for this lack of fundamental laboratory information is the challenge of spectroscopy in the THz regime combined with the challenge of studying unstable molecules. Our laboratory research involves characterization of astrophysically-relevant unstable species, including small radicals that are the products of photolysis reactions, organic ions formed via plasma discharges, and small reactive organics that form via O(¹D) insertion reactions. Our observational astronomy research seeks to examine the chemical mechanisms at play in a range of interstellar environments and to identify chemical tracers that can be used as clocks for the star-formation process. In this seminar, I will present recent results from our laboratory and observational studies that examine prebiotic chemistry in the interstellar medium. I will discuss these results in the broader context of my integrative research program that encompasses laboratory spectroscopy, observational astronomy, and astrochemical modeling.

PHYSICOCHEMICAL MODELS: SOURCE-TAILORED OR GENERIC? – Beatrice Kulterer, (Center for Space and Habitability, Universität Bern)

Physicochemical models can be powerful tools to trace the chemical evolution of a protostellar system and allow to constrain its physical conditions at formation. I will discuss whether source-tailored modelling is needed to explain the observed molecular abundances around young, low-mass protostars or if, and to what extent, generic models can improve our understanding of the chemistry in the earliest stages of star formation (Kulterer et al. 2020). The physical conditions and the abundances of nine simple and abundant molecules based on three models are compared. The physical models considered are 1D or 2D, the chemical networks consider two or three phases. After establishing the discrepancies between the calculated chemical output, the calculations are redone with the same chemical model for all three sets of physical input parameters. With the differences arising from the chemical models eliminated, the output is compared based on the influence of the physical model. Results suggest that the impact of the chemical model is small compared to the influence of the physical conditions, with considered timescales having the most drastic effect. Source-tailored models may be simpler by design; however, likely do not sufficiently constrain the physical and chemical parameters within the global picture of star-forming regions. Generic models with more comprehensive physics may not provide the optimal match to observations of a particular protostellar system, but allow a source to be studied in perspective of other star-forming regions.

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OCTOBER 2020 EVENT (WED 14 OCT, 2 PM EDT/11 AM PDT)

THE RIDDLE OF COMPLEX ORGANIC MOLECULES – Eric Herbst (Departments of Chemistry and Astronomy, University of Virginia)

In addition to stars, galaxies such as our own Milky Way contain interstellar matter, much of which is condensed into so-called interstellar clouds consisting of gas and nanoparticles known as dust grains. The clouds, ranging in size from a few to 100's of light years in extent, are of great interest as the ultimate sites of star and planetary formation. The denser clouds contain large numbers of molecules in the gas phase, mainly organic in nature, divided into classes of molecules dependent upon the age and physical nature of the clouds. Molecules are also observed in ice mantles of cold dust particles. Two distinctive classes of gaseous molecules are known as "carbon chains" and "complex organic molecules (COMs)". Carbon chains are exotic, very unsaturated, and often linear, whereas COMs resemble small terrestrial organic solvents, consisting of amines, alcohols, esters, etc. Until recently, based on observations, it was thought that carbon chains exist solely in cold dense clouds, whereas COMs exist in warmer regions in which star formation is occurring, known as "hot cores." More recently, COMs have been discovered in cold regions as well, introducing more complexity into our understanding of their chemistry. This talk will be concerned mainly with the local build-up chemistry thought to form COMs in both types of sources, and the degree of success that has been achieved.

FORMATION OF COMPLEX ORGANIC MOLECULES IN THE TRANSLUCENT CLOUD VIA TOP-DOWN PROCESSING ON DUST GRAINS – Ko-Ju Chuang,^* (Max Planck Institute for Astronomy)

Interstellar complex organic molecules (COMs) have been identified toward various star-forming regions from translucent clouds to the solar system. Interstellar sugar-like species (C_nH_2nO_n) have been intensively studied along with the bottom-up approaches; the ice chemistry scheme of CO-H₂CO-CH₃OH through "energetic" and/or "non-energetic" processing on dust grains. However, the icy origin of acetaldehyde and its (de-)hydrogenated derivatives (C₂H_nO), which are often observed in molecular clouds before CO freezes out, remains unclear. In this talk, I will present the laboratory study on the solid-state reactions that involve C₂H₂, which is one of the common hydrocarbon fragments of PAHs or hydrogenated carbonaceous dust (HAC) in the top-down scenario, and H/OH-radicals along with the H₂O formation/destruction sequence on grain surfaces under molecular cloud conditions. It is concluded that C₂H₂ readily acts as a molecular backbone providing a solid-state route for the formation of COMs, such as ketene (CH₂CO), acetaldehyde (CH₃CHO), vinyl alcohol (CH₂CHOH), ethanol (CH₃CH₂OH), and possibly acetic acid (CH₃COOH). The reaction network linking the above complex species described by the formula C₂H_nO is present.
^*Dr. Chuang received Honorable Mention in the 2020 competition for the Astrochemistry Subdivision Dissertation Award.

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SEPTEMBER 2020 EVENT (WED 9 SEP, 2 PM EDT/11 AM PDT)

THE MOLECULAR UNIVERSE – Alexander Tielens (Leiden Observatory, Leiden University and Astronomy Department, University of Maryland)

Over the last 20 years, we have discovered that we live in a molecular Universe: a Universe with a rich and varied organic inventory; a Universe where molecules are abundant and widespread; a Universe where molecules play a central role in key processes that dominate the structure and evolution of galaxies; a Universe where molecules provide convenient thermometers and barometers to probe local physical conditions. Understanding the origin and evolution of interstellar and circumstellar molecules is therefore key to understanding the Universe around us and our place in it and has therefore become a fundamental goal of modern astrophysics. The field is heavily driven by new observational tools that have become available over the last 20 years; in particular, space-based missions that have opened up the IR and submillimeter window at an ever-accelerating pace. Furthermore, our progress in understanding the Molecular Universe is greatly aided by close collaborations between astronomers, molecular physicists, astrochemists, spectroscopists, and physical chemists who work together in loosely organized networks. In this talk, I will sketch the progress that we have made over the last 20 years and outline some of the challenges that are facing us. The focus will be on understanding the unique and complex organic inventory of regions of star and planet formation that may well represent the prebiotic roots to life.

A PHOTOIONIZATION REFLECTRON TIME-OF-FLIGHT INVESTIGATION OF PHOSPHORUS CHEMISTRY IN EXTRATERRESTRIAL ICES – Andrew M. Turner,^* Cornelia Meinert, Ralf I. Kaiser (University of Hawaii at Manoa)

Multiple phosphorus-containing compounds have been detected in the Solar System (planetary atmospheres, comets, meteorites) along with interstellar and circumstellar environments. Of particular astrobiological interest are alkyl phosphonic acids (RH₂PO₃, R = methyl, ethyl, propyl, and butyl) extracted from the Murchison meteorite. These phosphonic acids are the only extraterrestrial phosphorus-containing organic compounds thus far discovered and offer a bioavailable and highly soluble form of phosphorus. This project investigates the synthesis of phosphorus-containing products of electron-irradiated interstellar ice analogues containing phosphine (PH₃), water (H₂O), carbon dioxide (CO₂), and hydrocarbons such as methane (CH₄). Phosphine is known to exist in circumstellar envelopes (IRC +10216), has been considered for comets (67P/Churyumov-Gerasimenko), and may serve as the phosphorus source of complex organic compounds such as the alkyl phosphonic acids. Utilizing in situ analysis techniques such as quadrupole mass spectrometry (QMS), tunable-photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), and infrared spectroscopy (FTIR) in addition to ex situ analysis by secondary-ion mass spectrometry (SIMS) and two-dimensional gas chromatography mass spectrometry (GCxGC-TOF-MS), the intermediates and products of these irradiated ice analogues are characterized to demonstrate the potential to synthesize organic phosphine-containing molecules in astrophysical environments. Notable results include phosphanes (P_xH_x+2), methylphosphanes (CH₃P_xH_x+1), and phosphorus oxoacids (H₃PO_x, x=1–4, and pyrophosphoric acid (H₄P₂O₇) along with their alkylated equivalents such as prebiotically significant methylphosphonic acid (CH₃P(O)(OH)₂) and methylphosphate (CH₃OP(O)(OH)₂).
^*Dr. Turner is the 2020 recipient of the Astrochemistry Subdivision Dissertation Award.

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The Astrochemistry Subdivision of the American Chemical Society sponsors a monthly seminar series that features the work of astrochemists from around the world, including senior experts in the field, postdocs, and students. The seminar series will feature both invited and contributed talks. To submit an abstract to be considered as a 15-minute contributed talk for a future event, please complete this form or email acs.astrochem@gmail.com.

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